The present invention relates generally to determining establishment causes and more specifically to methods and systems for determining radio resource control (RRC) establishment causes using non-access stratum (NAS) procedures.
As used herein, the terms “user equipment” and “UE” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other user agents (“UAs”) that have telecommunications capabilities. In some embodiments, a UE may refer to a mobile, wireless device. The term “UE” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station or other network node transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (EUTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems and equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with communication access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel to be used for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH) data transmissions, and sends scheduling information to the UEs through a control channel.
In existing telecommunications systems, the various signaling and protocol controllers that deliver telecommunication services are implemented in several protocol layers. Various peer to peer entities belonging to each of the layers signal and communicate with one another to enable and realize various functions so that services can be provided. Furthermore, each layer may provide one or more services to the upper layers. FIG. 1 is an illustration of some of the protocol layers found within existing telecommunications systems and illustrates a layered protocol that may be used for communications between a UE and a base station. As shown in FIG. 1, the network layers 12 reside above the access control layers 14. The network layers 12 and access control layers 14 may communicate with one another. Furthermore, because they reside above access control layers 14, the network control layers 12 receive services provided by the access control layers 14.
In a mobile communications network, the network layer signaling and protocol controllers of the UE and the core network (CN) communicate with one another through communications links established by the underlying radio access network (RAN) controllers. In UMTS and 3GPP terminologies, for example, the network layer between the UE and the CN is termed the Non Access Stratum (NAS). The radio access layer of the RAN is termed the Access Stratum (AS).
Because the underlying layers provide services to the upper layers, in the case of UMTS and 3GPP technologies, for example, the AS provides services to the NAS. One such service provided by the AS is to establish a signaling connection for the NAS of a UE such that the NAS of the UE can signal and communicate to an NAS of the core network. In long-term evolution/service architecture evolution (LTE/SAE) this service may be referred to as obtaining a signaling connection to access the enhanced packet core (EPC). To obtain the signaling connection, the AS executes an RRC connection establishment procedure. The procedure includes sending an RRC CONNECTION REQUEST message from the AS of the UE to the AS of the base station.
FIG. 2 is a flowchart showing an exemplary RRC establishment procedure executed by a UE in communication with an EUTRAN network. In a first step 20 the UE issues an RRCConnectionRequest message to the EUTRAN. In response, the EUTRAN sends an RRCConnectionSetup message to the UE in step 22 and receives an RRCConnectionSetupComplete message from the UE in step 24. A similar signaling procedure may be found in UMTS.
The RRC connection request procedure illustrated in FIG. 2 may be initiated by the RRC for its own needs, or the procedure can be initiated when the NAS transmits a request for a network connection to the AS such as to allow the NAS to communicate with the network. As such, the AS may request and establish resources on behalf of the NAS.
As part of the establishment of the signaling connection (e.g., as illustrated in FIG. 2), the RRC of the UE transmits to the AS of the base station an indication of the reason for requesting the connection. The reasons may include several values including emergency, highPriorityAccess, mt-Access, mo-Signalling, mo-Data, spare3, spare2, and spare1. Table 1 Illustrates example RRC signaling protocols including an establishment clause, and a definition of valid values for the establishment clause that may be provided by the NAS to the AS to request a signaling connection.
TABLE 1-- ASNI STARTRRCConnectionRequest ::=SEQUENCE { critical Extensions CHOICE {  rrcConnectionRequest-r8    RRCConnectionRequest-r8-IEs,  criticalExtensionsfuture   SEQUENCE { } }}RRCConnectionRequest-r84Es ::= SEQUENCE { ue-Identity  InitiaIUE-Identity, establishmentCause  EstablishmentCause, spare  BIT STRING (SIZE (1))}InitialUE-Identity ::=CHOICE { s-TMSI    S-TMSI, randomValue    BIT STRING (SIZE (40))}EstablishmentCause ::= ENUMERATED {emergency, highPriorityAccess, mt-Access, mo-Signalling,mo-Data, spare3, spare2, spare1}-- ASNISTOP
The establishment cause may indicate to the destination nodes (e.g., the base station/E-UTRAN and possibly the CN/EPC) the reason for such an establishment so that appropriate resources can be allocated for the signaling connection and subsequent usage of the signaling connection or the user plane connection. The establishment cause may also be used to discriminate/distinguish on charging tariffs/plans. In UMTS and EPS, the establishment cause that the RRC provides to the network in an RRC CONNECTION REQUEST message is taken from the inter-layer request from the NAS. As such, the RRC establishment cause that the AS (e.g., the RRC) uses in the RRC CONNECTION REQUEST is received from the NAS. Accordingly, it is the NAS that determines which establishment cause is to be used. For example, with reference to Table 1, “establishmentCause” may be used to provide the establishment cause for the RRC connection request as provided by the upper layers. With respect to the cause value names, highPriorityAccess relates to AC11 . . . AC15, ‘mt’ stands for ‘Mobile Terminating’ and ‘mo’ for ‘Mobile Originating.
In the case of an emergency call, the NAS initiating such an emergency call on behalf of the upper layers (e.g., the call applications) may indicate that an emergency call is being placed. If so, the RRC Establishment Cause may be read by the base station and the CN, and, in response, the base station and CN may be configured to do their utmost to provide and maintain resources for the emergency call.
In some network configurations, however, the UE may be configured to implement an IMS layer for packet switched (PS) communications (including voice and data communications). For the IMS layer within the UE there is a peer IMS layer on the CN side. The IMS layer within the base station resides above the NAS layer. On the UE side, the IMS sublayer of the UE sits on par with applications. As such, the IMS layer (or sublayer) is above the NAS, and above the Mobility Management functions and the Session Management functions. FIG. 3a is an illustration of layering within a UE showing the IMS sublayer. As shown, the IMS sublayer 30 resides above both NAS layer 32 and AS layer 34. The IMS layer may be used to initiate PS voice communications. In some cases, a user may wish to initiate an emergency voice communication using services provided by the IMS layer.
Various communications networks, including public land mobile networks (PLMNs), may be required to support a user making an emergency call. Generally, however, those networks do not support emergency calls placed within the PS domain (e.g., using IMS). As such, existing systems may rely upon circuit switched (CS) domain services to provide the emergency call. Even though a user's UE may be configured to provide voice communication using IMS, in the special case of an emergency call, the UE does not use the PS domain services provided by IMS. Instead, the UE switches to the CS domain service to place the emergency call. When the UE is connected to a network that does not provide CS domain services, for example LTE/SAE, the UE may be configured to implement CS fallback (CSFB) to provide emergency calls (see, for example, TS 3GPP 23.272). In CSFB, instead of using the PS domain, the UE is moved back to a 2G or 3G system and uses the CS domain of the 2G/3G system to place the emergency call.
Going forward, however, the 3GPP PS domain may be required to support emergency calls. In that case, because the PS domain of 3GPP uses IMS as the layer to setup, control and manage a call or session or transaction, it will be the IMS layer that realizes the PS domain emergency call. As such, to setup an emergency session, the IMS sublayer may trigger the NAS layer with a request to establish access to the EPC core. In response, the NAS may then setup an NAS signaling connection and the AS may setup the RRC connection. In turn, the EPC, upon responding to the NAS request for network access, sets up the necessary bearers to support the requested service. In existing networks, however, although the IMS layer may indicate that the requested resources are for an emergency call there is no existing mechanism for such an indication to be passed through the NAS to the AS and, consequently, to the base station or network. As such, after receiving the RRC connection request, the AS of the base station may be incapable of determining that a particular requested signaling connection is for an IMS session that is requested for an emergency call.
In some cases, a UE operating in a limited service state may be used to initiate an emergency call. A limited service state may result when a UE has no subscriber identity module (SIM), when a user has not paid his phone bill and has a suspended account, or when a user travels to a foreign country and attempts to access mobile services on a network that does not have an appropriate roaming agreement with the user's home provider. In those circumstances, when the UE is powered up, the UE may attempt to enter a state in which the UE can support an emergency call, but is unable to provide additional services. As such, the UE may camp on an available cell of the PLMN in a limited service state for the sole purpose of providing emergency calls. If, in that limited service state, the UE is configured to initiate PS domain voice services (e.g., via IMS) for the purpose of providing an emergency call, in many network configurations the AS of the base station may be incapable of determining that a particular IMS session requested by the UE in the limited service state is for an emergency call.
As such, it is difficult for a base station to determine that an RRC connection request received from a UE will ultimately be used for an IMS emergency call. If the base station cannot determine that the request is for an IMS emergency call, the base station is incapable of quickly establishing the emergency session by, for example, gracefully releasing lesser priority resources if no radio resources are available on the base station. These problems are exacerbated in network configurations implemented using a network sharing configuration wherein a RAN, base transceiver station (BTS) or base station is effectively shared between two or more core networks or PLMNs.